Hi there, @coveryourassets.

Yes, we can use QuickBooks Desktop with or without the internet. If any other tasks require internet access, like QuickBooks updates and third-party app integration, the software will direct you to connect to a secure internet connection.

Furthermore, I'm adding this article on how to manage your customer, vendors, and inventory items lists. This will also help you transfer your data from excel to your company: Add and edit multiple customers, vendors, and items. 

Please let me know if you have other concerns with your QuickBooks Desktop account. I'll be here to share  more information.